Metabolic and drug effects
Digoxin effect and toxicity
The sagging 'reverse tick' ST segment of therapeutic digoxin — and the arrhythmias that mean toxicity.
64 bpmSinus
What you see
- Downsloping 'scooped' ST depression, most obvious in leads with tall R waves.
- Flattened, inverted or biphasic T waves.
- Shortened QT interval.
- Slight PR prolongation and prominent U waves.
Why it looks like that
Digoxin inhibits the Na⁺/K⁺-ATPase, raising intracellular calcium and shortening ventricular repolarisation, while increasing vagal tone at the AV node. The ST change is a pharmacological effect present at therapeutic levels — it is not ischaemia and not toxicity.
What to do
- Recognise digoxin effect and do not stop the drug for it.
- Toxicity is a different picture: nausea, confusion, xanthopsia, and arrhythmias — the classic being atrial tachycardia with block, and bidirectional VT.
- Treat toxicity with digoxin-specific antibody fragments; correct hypokalaemia and hypomagnesaemia.
- Avoid calcium in digoxin toxicity with hyperkalaemia — the 'stone heart' concern, though the evidence is weaker than the teaching.
The trap
The reverse tick is routinely misread as ischaemic ST depression, and digoxin gets stopped. Conversely, toxicity is missed because the level is 'normal' — toxicity is clinical, and hypokalaemia, renal impairment, amiodarone, verapamil and age all lower the threshold at which it occurs.
reverse tickDigiFabatrial tachycardia with blockbidirectional VT